In the conventional art, reflected waves are reduced by adjusting an output impedance to eliminate an impedance mismatch occurring in a sender circuit, for example. Specifically, an output circuit for varying an output impedance in accordance with a control signal, a dummy transmission line connected to the output circuit and equivalent to an actual load transmission line, a voltage detecting circuit connected to the other end of the dummy transmission line and for detecting an initial voltage amplitude from the output circuit, and an impedance control signal generating circuit for generating a control signal for controlling the output impedance based on the detected voltage value (see Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-17518